Related to future statutory provisions regarding the nitric oxide emission of motor vehicles, an adequate exhaust gas treatment is required. In order to store the arising nitric oxides a nitric oxide storage catalyst can be used. The storage catalyst can only store a limited amount of nitric oxides and has to be regenerated when the maximum storage capacity is reached at the latest. A procedure for regenerating is based on the introduction of an over-stoichiometric fuel-air-mixture, which is enriched with fuel, for a specific period of time into the combustion engine, and on the addition of the developed “rich” exhaust gas over the storage catalyst. The carbon monoxide, which is contained in the rich exhaust gas, and the hydrocarbons, which are also contained, are oxidized by the nitric oxides, that are present in the storage catalyst, into carbon dioxide and water. The emerging nitrogen is brought away with the exhaust gas. The end of the regeneration is typically thereby determined, that rich exhaust gas leaves the storage catalyst without reacting. This is detected by the linear or binary signal.
Alternatively to the controlling of the fuel-air-mixture in the direction of a higher fuel concentration, the regeneration can also take place by a direct addition of a reduction agent into the exhaust gas system.
Due to DE 197 39 848 A1 a possibility is known, to determine the end of the phase with rich exhaust gas. Therefore a probe is arranged in the exhaust gas after the storage catalyst, which measures oxygen. As soon as the probe notices a transfer from a lean to a rich mixture, this means, that the storage catalyst does not provide sufficient oxygen for the oxidization of the hydrocarbons and the carbon monoxides and that it is discharged from nitric oxides, which contain oxygen. As a result the phase can be terminated with oxygen deficiency and be redirected back to the phase with a lean mixture (oxygen surplus). Since the transfer from lean exhaust gas to rich exhaust gas takes place after the storage catalyst, a breakthrough of carbon monoxide and hydrocarbons has to be accepted at the termination of the regeneration phase, in order to completely empty the catalyst from stored nitric oxides. Undesired carbon monoxide- and hydrocarbon-emissions occur, which can cause problems with the abidance of the contaminant marginal values. Furthermore the fuel consumption is raised over the needed dose by the extension of the regeneration phases.
Also from U.S. Pat. No. 5,775,099 a procedure for the exhaust gas purification of a combustion engine becomes known. The exhaust gases of the combustion engine are added through the storage catalyst. At a lean air-fuel-relation the exhaust gas shows a high nitric oxide concentration, which is stored in the storage catalyst. At a stoichiometric or rich air-fuel-relation the nitric oxide from the storage catalyst is reduced to nitrogen. For this purpose U.S. Pat. No. 5,775,099 suggests to increase the fuel amount, so that a stoichiometric or rich air-fuel-relation results. Thereby fuel is injected at the end of the compression stroke in such a quantity, that a lean middle air-fuel-mixture evolves, and additionally fuel is injected during the intake stroke, so that a stoichiometric or rich air-fuel-relation evolves. The exhaust gas, that has developed thereby, delivers the needed components for the regeneration of the storage catalyst.
A disadvantage of the described methods is that a part of the reduction agent, which is contained in the rich exhaust gas, already leaves the storage catalyst, before it is completely regenerated. This conversion rate of the storage catalyst depends highly on the temperature of the storage catalyst's surface, the exhaust gas volume flow, the filling size and the ageing of the storage material. The dosage of the reduction agent has to be aligned with the mentioned influencing factors.
It is the invention's task to provide a method for regenerating an emission control system, which doses the reduction agent so, that no unneeded excess consumption takes place and therefore the environment is not unnecessarily burdened.
The invention's further task is to provide a corresponding device.